1. Field of the Invention
The present invention relates to a control apparatus for a brushless DC motor including a rotor having a permanent magnet, and a stator that generates a rotating magnetic field for rotating the rotor.
Priority is claimed on Japanese Patent Application No. 2003-140726, filed May 19, 2003, the content of which is incorporated herein by reference.
2. Description of the Related Art
Vehicles are known, having a power source for driving a vehicle, including a brushless DC motor using a permanent magnet as a field magnetic, such as fuel cell vehicles, electric vehicles, or hybrid vehicles.
A known control apparatus for such a brushless DC motor is a control apparatus that measures a phase current supplied to each phase of a brushless DC motor, converts a measurement value of the phase current to a d-axis current and a q-axis current on orthogonal coordinates rotating in synchrony with a rotor. For example, d-q coordinates, with a direction of a magnetic flux of the rotor in a d-axis (a field axis) and a direction orthogonal to the d-axis in a q-axis (a torque axis), performs feedback control so that a difference between a command value and the measurement value of the current becomes zero on the d-q coordinates.
Specifically, from each difference between the command value and the measurement value on the d-q coordinates, that is, a d-axis current difference and a q-axis current difference, a d-axis voltage command value and a q-axis voltage command value on the d-q coordinates are calculated by a PI action or the like. Then, from the d-axis voltage command value and the q-axis voltage command value, each voltage command value is calculated with respect to a phase voltage supplied to each phase of the brushless DC motor, for example, three phases: a U-phase, a V-phase, and a W-phase. Then, each voltage command value is input as a switching command to an inverter constituted by a switching element such as a transistor, and AC power for driving the brushless DC motor is output from the inverter according to the switching command.
In such a control apparatus, information on an rotation angle of the rotor, that is, a magnetic pole position of the rotor is required in a coordinate conversion processing or the like of the current, and sensorless control is known that omits a position measuring sensor for measuring the rotation angle, and estimates the rotation angle of the rotor based on an induced voltage relating to the magnetic pole position of the rotor (for example, see “'93 Motor Technology Symposium, '93 Motor General Session” by Matsui, Japan Management Association, Apr. 16, 1993, B4-3-1 to B4-3-10”).
In the sensorless control of the brushless DC motor according to an example of the related art, first, based on a circuit equation on γδ coordinates in synchrony with an estimated rotation angle of the rotor having a phase difference Δθ with respect to the d-q coordinates in synchrony with an actual rotation angle of the rotor, the fact that the phase difference Δθ can be approximated by a sine value (sin Δθ) of the phase difference Δθ (sin Δθ≈Δθ) when the phase difference Δθ is sufficiently small is used to calculate a sine component of an induced voltage including the sine value (sin Δθ) of the phase difference Δθ.
Then, a rotational angular velocity of the rotor when the phase difference Δθ is zero is corrected by a value obtained by controlling and amplifying the sine component of the induced voltage by, for example, a PI (proportional integral) action, a value obtained by the correction is set as a rotational angular velocity on the γδ axes, and then the rotational speed on the γδ axes is time-integrated to estimate the rotation angle of the rotor.
However, if there is an error in an inductance component value or the like in the circuit equation on the γδ coordinates that calculates the sine component of the induced voltage, the error in the sine component of the induced voltage increases in proportion to the angular velocity of the rotor. Thus, for the rotation angle of the rotor estimated based on the PI action with respect to the sine component of the induced voltage, a larger revolution speed of the brushless DC motor causes more errors.
Particularly, when the brushless DC motor is included as a drive source of a vehicle, the angular velocity of the rotor often varies significantly, and frequent loss of synchronization impairs traveling of the vehicle.